1. Field of the Invention
The present invention relates generally to a torque sensor for monitoring torque exerted on a shaft or axle of an object. More specifically, the invention relates to a magnetostriction type torque sensor.
2. Description of the Background Art
A typical construction of a magnetostriction type torque sensor, known in the prior art, generally comprises a shaft member to measure torque exerted thereto, and an electromagnetic coil provided in the proximity of the shaft, defining a small gap between the opposing surfaces, and having a yoke made of material having a high permeability material, such as permalloy. A magnetic field is thus generated around the shaft by energizing the electromagnetic coil for monitoring magnetostriction component by means of a detector coil, which is also wound on the yoke.
In the practical measurement of torque exerted on the shaft member, electric power is supplied to the electromagnetic coil. The energized electromagnetic coil generates magnetic flux between the shaft and the yoke to form a magnetic circuit. The detector coil is placed in the magnetic circuit. The permeability of the shaft member is variable depending upon the magnetostriction therein. Therefore, the density of the magnetic flux to be detected by the electric power induced in the detector coil varies depending upon the magnetostriction in the shaft member.
If such a magnetostriction type torque sensor is used for monitoring torque exerted on an axle in an automotive power train, such as a drive shaft, a column shaft, etc., the variation of the generated power in the detector coil tends to become smaller than that required for accurate measurement. Namely, the axle in the power train is generally formed of steel for machine structural use, such as that produced according to JIS SC, SCr, SCM, SNCM and so forth. Such an axle is provided high strength in order to have low sensitivity to variations of the exerted torque. Therefore, the sensitivity of the torque sensor becomes too low to expand hysteresis range for lowering accuracy in measurement of the measured torque.